PostgreSQL is implemented
using a simple "process per user"
client/server model. In this model there is one client process connected to exactly one server process. As we do not know ahead of time
how many connections will be made, we have to use a master process that spawns a new server process
every time a connection is requested. This master process is
called postgres and listens at a
specified TCP/IP port for incoming connections. Whenever a
request for a connection is detected the postgres process spawns a new server process. The
server tasks communicate with each other using semaphores and shared
memory to ensure data integrity throughout concurrent data
access.

The client process can be any program that understands the
PostgreSQL protocol described in
Chapter 46. Many clients are based on
the C-language library libpq,
but several independent implementations of the protocol exist,
such as the Java JDBC
driver.

Once a connection is established the client process can send a
query to the backend (server). The query
is transmitted using plain text, i.e., there is no parsing done
in the frontend (client). The server
parses the query, creates an execution
plan, executes the plan and returns the retrieved rows to the
client by transmitting them over the established connection.